The subject matter of the present invention pertains to means within a raster-scan type display system for producing an image with enhanced gray scale quality.
In a conventional raster-scan display system, for example a television receiver or a computer display terminal, the image formed on the faceplate of the display device is composed of successive presentations of short lived image frames, each of which comprises one or more image fields. Each field or frame is composed, in turn, of a plurality of sequentially occuring, mutually parallel raster lines.
In a television environment, each frame comprises two fields of 2621/2 raster lines presented at a rate of 60 fields, or 30 frames, per second. The raster lines of the two fields forming each frame are spacially interlaced at display time to produce a composite frame of 525 lines. This interlacing of the raster lines permits an image with acceptable resolution to be produced at a rate sufficient to eliminate perceptable flicker that might otherwise be caused by the successive presentation of the short lived images.
In certain computer-generated-display environments, especially those wherein the display device is employed to produce a hard copy of the image being formed, each frame of the input signal may comprise a single field and be displayed without being interlaced.
In either case, maximum resolution of the image being formed is achieved when the width of the individual raster lines constituting the image is maintained at a minimum. However, as the spaces between the raster lines contain no information, minimizing the raster line width also minimizes ability to produce an image with acceptable gray scale quality. In a television environment, a balance is struck between resolution and gray scale quality by defocusing slightly the electron beam producing the raster lines so as to increase the line width and minimize the effect of the blank spaces between the lines. This approach is generally not acceptable in a computer environment, however, because of the requirement for high resolution.
Accordingly, a need exists for a raster-scan display system capable of forming an image characterized by both high resolution and increased gray scale quality.